old_file_name = input("请输入要备份的文件名: ")
"""   123.txt """
"""123[复件].txt"""

index = old_file_name.index('.')

new_file_name = old_file_name[0:index] + '[复件]' + old_file_name[index:]

print(new_file_name)

with open(old_file_name, 'r') as old_f:
    with open(new_file_name, 'w') as new_f:
        new_f.write(old_f.read())
